Photochemistry of conjugated polyacetylenes

Abstract

Several aryl 1,3,5-hexatriynes were synthesized by the use of 1-aryl-4-trimethylsilyl-1,3-butadiyne intermediates. 1-Phenyl-1,3,5-heptatriyne and 1-(1``-naphthyl)-1,3,5-heptatriyne were prepared by the use of 1-trimethylsilyl-1,3-pentadiyne which is a very stable precursor for the 1,3-pentadiyne at room temperature. 1-Phenyl-6-tbutyldimethylsilyl-1,3,5-hexatriyne was synthesized by Cadiot-Chodkiewicz coupling reaction and 1-phenyl-1,3,5-hexatriyne was synthesized by desilylation of 1-phenyl-6-tbutyldimethylsilyl-1,3,5-hexatriyne. Fluorescence quantum yields and phosphorescence lifetimes were measured for the synthetic polyacetylenes. 1-Aryl-1,3butadiynes, $\underline{1}-\underline{6}$ were photolyzed in 2,3-dimethyl-2-butene to obtain [2+2] type photocycloadducts $\underline{16}-\underline{21}$ except for $\underline{5}$ which yields two photoadducts. Azulene quenching, fluorescence, and exciplex emission studies suggest for $\underline{1}-\underline{4}$ that the reaction proceeds from both singlet and triplet excited states but not through the singlet exciplex formation with olefins, and for 5 and 6 the reaction proceeds from the triplet excited state. Photolysis of 1-phenyl-1,3,5- hexatriynes, $\underline{9}-\underline{12}$ in deaerated 2,3-dimethyl-2butene gave bicyclopropyl photoadducts, $\underline{23}-\underline{27}$. No photoadduct was formed with aerated 2,3-dimethyl-2-butene or deaerated solutions of dimethyl fumarate, methyl crotonate, dimethyl maleate, $\underline{\mbox{trans}}$-1,2-dichloroethylene, and acrylonitrile, while, 1,6-diphenyl1,3,5-hexatriyne in deaerated 2,3-dimethyl-2-butene/acrylonitrile (1:1 molar ratio) cosolvent system yielded the photoadducts $\underline{23}$ and $\underline{28}$. A plausible reaction mechanism involving electron delocalized triplet state (1,6-diradical type) and carbene intermediate was proposed. The irradiation of -naphthyl-1, 3-butadiynes, $\underline{2}-\underline{4}$ in aqueous sulfuric acid yields two photohy...